XTLS Vision: Serialize VisionWriter writes from concurrent mux sessions (#7337)

https://github.com/XTLS/Xray-core/pull/7337#issuecomment-6692618802

Fixes https://github.com/XTLS/Xray-core/issues/7336

---------

Co-authored-by: 风扇滑翔翼 <Fangliding.fshxy@outlook.com>
Co-authored-by: RPRX <63339210+RPRX@users.noreply.github.com>

proxy/proxy.go

@@ -13,6 +13,7 @@ import (
 	"math/big"
 	"runtime"
 	"strconv"
+	"sync"
 	"time"
 
 	"github.com/pires/go-proxyproto"
@@ -301,6 +302,7 @@ type VisionWriter struct {
 	// internal
 	writeOnceUserUUID  []byte
 	directWriteCounter stats.Counter
+	access             sync.Mutex
 
 	testseed []uint32
 }
@@ -324,6 +326,9 @@ func NewVisionWriter(writer buf.Writer, trafficState *TrafficState, isUplink boo
 }
 
 func (w *VisionWriter) WriteMultiBuffer(mb buf.MultiBuffer) error {
+	w.access.Lock()
+	defer w.access.Unlock()
+
 	var isPadding *bool
 	var switchToDirectCopy *bool
 	var spliceReadyInbound *session.Inbound